The present invention relates to an optical glass, a shapeable material formed of the above optical glass, a process for the production of the shapeable material, an optical product formed from the above optical glass as a raw material and a process for the production of the above optical product. It particularly relates to a high-dispersion or intermediate-dispersion optical glass having high refractive index, a precision-press shapeable material formed of the above optical glass, a process for the production of the shapeable material, an optical product formed from the above optical glass as a raw material and a process for the production of the above optical product.
For forming a desired optical system, generally, it is required to combine various optical elements having different refractive indices nd and Abbe numbers xcexdd. For this purpose, various optical glasses having different refractive indices nd and Abbe numbers xcexdd have been developed.
For example, as a high-dispersion or intermediate-dispersion optical glass having refractive indices nd of approximately 1.7 or greater and Abbe numbers xcexdd of approximately 28 to 41 (the above optical glass will be referred to as xe2x80x9ca high-refractivity high-dispersion or intermediate-dispersion optical glassxe2x80x9d or xe2x80x9chigh-refractivity high- or intermediate-dispersion optical glassxe2x80x9d hereinafter), various glasses classified into dense barium flint glass, dense flint glass, lanthanum flint glass, dense lanthanum flint glass or the like have bee developed (see JIS Z8120).
Meanwhile, precision-press molding is widely used as a method of forming optical products such as optical elements (a lens, a prism or the like) and optical fiber fixing members from a glass. In precision-press molding for producing a shaped article of a glass, a precision-press shapeable material (glass preform) is shaped into a shaped article of glass having a form of an end product or a form close thereto, in a shaping mold having cavity of a predetermined form at a high temperature under a high pressure. Shaping surfaces of the shaping mold used for the precision-press shaping have a high surface accuracy. The above precision-press shapeable material (glass preform) is pressed in the above shaping mold when the above shaping mold and the precision-press shapeable material (glass preform) have temperatures in a predetermined temperature range, whereby the shape of each of the above shaping surfaces is transferred to the precision-press shapeable material (glass preform).
When a shaped article of a glass is produced by the precision-press shaping, it is required to shape a precision-press shapeable material under pressure at a high temperature as described above. The shaping mold used therefor is exposed to a high temperature, and a high pressure is exerted thereon. Concerning the precision-press shapeable material, therefore, (1) it is desired to decrease the sag temperature Ts thereof to make it as low as possible for preventing the damage of the shaping mold per se and a release film provided on an internal surface of the shaping mold, which damage is caused by a high-temperature environment during the press shaping, (2) it is also desired to decrease the reactivity thereof with a shaping mold to make it as low as possible for preventing the downgrading of surface properties of a shaped article, which downgrading is caused by a reaction product formed by a reaction between the precision-press shapeable material and the shaping mold, and further, (3) it is desirable that the precision-press shapeable material contains no lead oxide (PbO) in view of environmental protection.
A high-refractivity high-dispersion or intermediate-dispersion optical glass which is preferred as a precision-press shapeable material in view of the above points (2) and (3) is a high-refractivity high-dispersion or intermediate-dispersion optical glass which contains no lead oxide as an essential component, such as an optical glass classified as lanthanum flint glass or dense lanthanum flint glass. A high-refractivity high- or intermediate-dispersion optical glass which contains lead oxide as an essential component, such as an optical glass classified as dense barium flint glass or dense flint glass, cannot be said to be preferred as the above precision-press shapeable material in view of the above points (2) and (3).
In general, the high-refractivity high- or intermediate-dispersion glass containing no lead oxide as an essential component is borate glass or borosilicate glass. As the above glass, there are known an optical glass of B2O3xe2x80x94SiO2xe2x80x94Li2Oxe2x80x94CaOxe2x80x94La2O3xe2x80x94TiO2xe2x80x94ZrO2xe2x80x94Nb2O5 system disclosed in JP-A-61-232243 and an optical glass of SiO2xe2x80x94B2O3xe2x80x94Li2Oxe2x80x94La2O3xe2x80x94ZrO2xe2x80x94Nb2O5 system disclosed in JP-A-61-146730.
There are a variety of methods of forming press-shapeable material used for precision-press shaping, depending upon kinds of shaped articles as end products. In any method, however, it is required to prepare a desired glass melt in a melting furnace, introduce the glass melt to a desired site through a flow pipe and flow it down through a flow outlet of the flow pipe.
As a material for the above melting furnace or the flow pipe, generally, Pt (platinum) or a Pt alloy is used. A conventional high-refractivity high- or intermediate-dispersion optical glass containing no lead oxide as an essential component, i.e., a conventional high-refractivity high- or intermediate-dispersion optical glass formed of borate glass or borosilicate glass is easily wettable with the Pt or Pt alloy used as a material for the melting furnace or the flow pipe during the melting of the glass.
When a glass melt is easily wettable with a flow pipe formed of Pt or a Pt alloy, as shown in FIG. 1, there occurs a phenomenon that part 2a of glass melt 2 flowing down from a flow pipe 1 causes xe2x80x9cwetting backwardxe2x80x9d (flows backward) along an external surface of an end portion of the flow pipe 1 on its outlet side even if the flow pipe 1 is disposed so as to direct the outlet downwardly (the above phenomenon will be referred to as xe2x80x9cwet backward phenomenonxe2x80x9d hereinafter). The wet backward phenomenon intensifies with a decrease in the diameter of the flow pipe 1.
When it is attempted to produce a precision-press shapeable material for a small-sized optical product, e.g., a precision-press shapeable material for a small-diametered lens, the above flow pipe 1 necessarily has a small diameter, so that the above conventional high-refractivity high- or intermediate-dispersion optical glass containing no lead oxide as an essential component comes to exhibit an intensified wet backward phenomenon when it is used for obtaining the above precision-press shapeable material for a small-sized optical product. When the above wet backward phenomenon intensifies, there are caused the following problems. (i) When a glass melt is flowed down through a flow pipe to obtain a glass gob, the glass gob has striae, and as a result, an optical product having desired optical properties can be no longer obtained from the glass gob. (ii) When a glass melt is flowed down through a flow pipe to obtain a glass gob, it is difficult to adjust the weight of the glass gob, and as a result, it is difficult to adjust the weight of an optical product to be obtained from the glass gob, or there is required an step of adjusting the weight of the optical product. In FIG. 1, reference numeral 3 indicates a flowing furnace for heating the flow pipe 1.
It is a first object of the present invention to provide an optical glass which is almost free from the above wet backward phenomenon and which is a high-refractivity and high- or intermediate dispersion optical glass.
It is a second object of the present invention to provide a precision-press shapeable material which can easily give a small-sized precision-press shapeable material of a high-refractivity and a high- or intermediate dispersion optical glass, and a process for the production thereof.
It is a third object of the present invention to provide a optical product which can easily give a small-sized optical product formed of a high-refractivity and a high- or intermediate dispersion optical glass, and a process for the production thereof.
According to the present invention, the above first object of the present invention is achieved by an optical glass comprising silicon oxide and boron oxide and having a refractive index (nd) of at least 1.7 and an Abbe number (xcexdd) of 28 to 41, the ratio of a content of the silicon oxide to a content of the boron oxide being greater than 0.78, the optical glass having a contact angle of at least 40xc2x0 to Pt or a Pt alloy at a predetermined temperature equivalent to, or higher than, its liquidus temperature or in a predetermined temperature range whose lower limit is equivalent to, or higher than, the liquidus temperature and having a sag temperature Ts of 580xc2x0 C. or lower (the above optical glass will be referred to as xe2x80x9coptical glass Ixe2x80x9d hereinafter).
According to the present invention, the above first object of the present invention is also achieved by another optical glass comprising silicon oxide, boron oxide, lithium oxide, calcium oxide, titanium oxide and niobium oxide, the total content of silicon oxide, boron oxide, lithium oxide, calcium oxide, titanium oxide and niobium oxide being at least 63% by weight, the content of the silicon oxide being 17 to 33% by weight (exclusive of 17% by weight and 33% by weight), the content of the boron oxide being 1 to 25% by weight, the content of the lithium oxide being 5 to 11% by weight, the content of the calcium oxide being 5 to 27% by weight (exclusive of 5% by weight), the content of the titanium oxide being 1 to 20% by weight, the content of the niobium oxide being 13 to 30% by weight (exclusive of 13% by weight), the total content of the silicon oxide and the boron oxide being 20 to 50% by weight, the ratio of the silicon oxide content to the boron oxide content being greater than 0.78, the optical glass further comprising 0 to 16% by weight, exclusive of 16% by weight, of lanthanum oxide, 0 to 12% by weight of zinc oxide, 0 to 15% by weight of barium oxide, 0 to 10% by weight of zirconium oxide, 0 to 10% by weight of strontium oxide, 0 to 6% by weight of tungsten oxide, 0 to 7% by weight of aluminum oxide, 0 to 5% by weight of sodium oxide, 0 to 5% by weight of potassium oxide, 0 to 5% by weight of yttrium oxide, 0 to 5% by weight of gadolinium oxide, 0 to 5% by weight of ytterbium oxide, 0 to 5% by weight of tantalum oxide, 0 to 2% by weight of arsenic oxide and 0 to 2% by weight of antimony oxide (the above optical glass will be referred to as xe2x80x9coptical glass IIxe2x80x9d hereinafter).
According to the present invention, the above second object of the present invention is achieved by a precision-press shapeable material formed of the above optical glass of the present invention.
Further, the process for the production of the above precision-press shapeable material comprises dropping a glass melt for the above optical glass of the present invention from an outlet of a flow pipe made of Pt or a Pt alloy to form a glass gob and shaping the glass gob to obtain the precision-press shapeable material.
According to the present invention, the above third object of the present invention is achieved by an optical product formed of the above optical glass of the present invention.
Further, the process for the production of the above optical product comprises the steps of placing a precision-press shapeable material produced by the above process for the production of a precision-press shapeable material, provided by the present invention, in a shaping mold comprising at least an upper mold member and a lower mold member for forming a predetermined cavity form, and precision-press shaping the above precision-press shapeable material into a predetermined form with the above shaping mold while the precision-press shapeable material is a softened state under heat.